Most Pioneer head units have a connecter called an IP-BUS. The Pioneer IP-Bus lets you connect things like XM radio, cd changer, or an aux input adapter (you will have to buy the adapter ~$30-40). It looks like this particular model lacks the IP-Bus connecter.
The next cheapest model that has an IP-Bus connecter is the DEH-4400
http://www.pioneerelectronics.com/Pi...,21414,00.html